1. Field of the Invention
This invention relates generally to a method of manufacturing a multilayer material used for production of plain bearings or the like, and more particularly to such a method of manufacturing the multilayer material including a metal plate which is bonded to a back metal and has a directional dendritic structure.
2. Description of the Related Art
A method of manufacturing a multilayer material for bearings in which a copper bearing alloy (bearing alloy layer) is bonded to a back metal such as steel plate includes a sintering method and a continuous casting method. In the sintering method, sintered powder of copper alloy composing the copper bearing alloy is distributed over the steel plate so as to have a predetermined thickness. Subsequently, the steel plate is heated at 850° C. to 900° C. in a reducing atmosphere (“primary sintering”). The steel plate is then rolled so that the density of a copper alloy powder layer becomes 100%. Finally, a second sintering is carried out under the same condition as the primary sintering, whereupon a multilayer material is completed.
In the continuous casting, both ends of a steel plate are bent into a generally L-shape so that the plate is formed into the shape of a channel. Subsequently, the channel-shaped steel plate is preheated up to 1000° C. in a reducing atmosphere, and molten copper alloy is poured into the channel-shaped steel plate. Oil quenching is carried out at the underside of the steel plate so that the molten copper alloy poured is cooled from a lower part thereof, whereby solidification progresses upward. The L-shaped portions of the steel plate are then cut off, and an unnecessary portion of the copper alloy surface is also cut off. Subsequently, the martensitic steel plate is heated until 800° C. to be softened, whereupon the multilayer material is completed.
The above-described sintering method necessitates equipment both for the primary and secondary sintering and rolling equipment. The continuous casting method requires equipment for forming the steel plate into the channel-shape, preheating equipment and casting equipment. Accordingly, each method requires an extremely long production line.
Furthermore, in the case of the sintering method, holes tend to remain in the structure of the copper alloy layer. Thus, an insufficient densification in the structure of the copper alloy layer results in a problem of low fatigue resistance. Furthermore, a bonding strength is low between the copper alloy layer and the steel plate and further, heating in the secondary sintering coarsens the structure of the copper alloy layer and reduces the strength of the layer. On the other hand, in the case of the continuous casting method, the bonding strength is high between the copper alloy layer and the steel plate. However, the steel plate is hardened when the oil quenching is carried out at the underside of the steel plate so that the molten copper alloy poured is solidified. Annealing is accordingly required afterwards so that the hardened steel is softened. The annealing coarsens the structure of the copper alloy layer and reduces the strength of the layer.